In a wireless system such as an HSDPA system, each mobile station frequently reports wireless channel quality (hereafter called a channel quality indicator (CQI)) to a base station so that the base station can perform link adaptation, e.g., transmission power control, adaptive modulation and coding (AMC), and fast packet scheduling among users, under fast fading environments.
For example, in an HSDPA system, the mobile station measures Ec/I0 (receiver chip energy to interference power ratio) of a common pilot channel transmitted from the base station, quantizes the measured Ec/I0 to 32 levels, and sends the quantized value as a CQI to the base station at intervals of 2 ms (or its integral multiple).
The base station compares CQIs sent from all mobile stations within its coverage area and selects a user whose data are to be transmitted in the next radio frame, for example, by performing fast packet scheduling.
Meanwhile, to improve frequency efficiency, the base station is preferably configured to select a user with a high CQI, i.e., a user in a good wireless channel condition. Therefore, in many systems, packet scheduling is performed taking into account CQI values.
As scheduling methods, maximum carrier-to-interference power ratio (Max C/I) and proportional fairness (PF) are widely used. In Max C/I, a user with the maximum CQI is always selected as long as there are transmission data. Accordingly, Max C/I has a disadvantage that it cannot select users in an equal manner. The method of improving frequency efficiency by selecting a user with a high CQI from multiple users is called multi-user diversity.
A user (i) selected by Max C/I is represented by formula (1) below.i=arg maxqj(t)  (1)
wherein qj(t) represents the current CQI (instantaneous CQI) of the j-th user.
On the other hand, a user (i) selected by PF is represented by formula (2) below.i=arg max(rj(t)/Rj(t))  (2)
wherein rj(t) represents a transmission rate corresponding to qj(t), and Rj(t) represents a throughput up to the present time of the j-th user.
In formula (2), rj(t) increases as qj(t) increases. In other words, the better the propagation condition is, the larger rj(t) is. Thus, PF is a scheduling method that takes into account CQI values.
Formula (3) represents a variation of PF where CQIs are directly used.i=arg max{qj(t)−Qj(t)}  (3)
wherein Qj(t) represents a mean value of qj(t).
A PF scheduler can achieve some degree of equality among users and provide high frequency efficiency. For this reason, many schedulers that can achieve more equality among users or that take into account delay time in a real-time service, such as a modified largest weighted delay first (M-LWDF) scheduler, are proposed based on the PF scheduler. Any of such schedulers takes into account CQI values.